This invention relates to vulcanizable ethylene-vinyl acetate compositions suitable for injecting molding, a process for preparing crosslinked thermoset injection molded articles which employs said compositions, and to crosslinked thermoset articles of manufacture prepared therefrom.
The successful manufacture of thermoset injection molded articles requires the use of vulcanizable (crosslinkable) compositions that will not only satisfy the performance requirements of the final article product, but which will also meet the stringent requirements of thermoset injection molding. For instance, thermoset injection molding imposes specific processing conditions on the vulcanizable composition such as stability in the machine barrel for long periods of time (i.e., the avoidance of premature crosslinking), rapid cure times in the heated mold, sufficient flow to fill complex parts, low shrinkage to avoid deformation when thick and thin sections are present and, very importantly, the thermoset molded product must have a sufficient tensile strength at the mold temperature to survive the demolding operation without tearing or distorting. For these reasons, formulations employed in the extrusion coating of a wire or cable and formulations for thermoset injection molding may be widely different and are usually not interchangeable for the two processes.
Heretofore, the thermoset injection molded articles have been made for the most part from elastomeric materials such as ethylene-propylene diene monomers. However, such thermoset EPDM rubbers generally require long cure cycle times at high temperatures and normally require a surface treatment in order to be painted.
It has now been discovered that thermoset injection molded articles can be prepared from the vulcanizable (crosslinkable) ethylene-vinyl acetate compositions of this invention which compositions not only require relatively low temperature short cure cycle times but in addition have good molding latitudes, i.e. are stable in the machine barrel for long periods of time, and also result in thermoset products that have good low temperature impact resistance, a good secant modulus range, good paint adhesion, low shrinkage, high temperature stability and a tensile strength at the mold temperature of at least about 250 pounds per square inch allowing for an easy removal of the thermoset injection molded article from the hot mold during the demolding operation without tearing or distorting said article.
Thus, it is an object of this invention to provide a vulcanizable ethylene-vinyl acetate composition which is particularly useful in the manufacture of thermoset injection molded articles. Another object of this invention is to provide a process for preparing thermoset injection molded articles which comprises employing as the vulcanizable composition, the above mentioned novel composition of this invention. A further object of this invention is to provide an article of manufacture comprising the crosslinked thermoset product obtained upon injection molding and crosslinking said vulcanizable ethylene-vinyl acetate composition of this invention. Other objects and advantages of this invention will become readily apparent from the following description and appended claims.
More specifically, one aspect of the instant invention may be described as a vulcanizable composition suitable for thermoset injection molding consisting essentially of about 35 to about 75 parts by weight of (A) an ethylene-vinyl acetate copolymer containing about 7 to about 55 weight percent of vinyl acetate based on the total weight of said copolymer; about 65 to 25 parts by weight of (B) a clay filler selected from the class consisting of an organosilane treated clay and an organopolyester treated clay; the sum of (A)+(B) being about 100 parts by weight in the composition; about 1 to about 5 parts by weight based on the sum of (A)+(B) of (C) an organic peroxide crosslinking agent having a half-life at its stock temperature of greater than one hour; and 0 to about 5 parts by weight based on the sum of (A)+(B) of (D) of an organic crosslinking booster; wherein said components (A), (B), (C) and (D) are such that a crosslinked product obtained upon crosslinking an admixture containing only said components (A), (B), (C) and (D) in the same proportions as (A), (B), (C) and (D) are present in said vulcanizable composition, has a hot tensile strength of at least 250 pounds per square inch when measured according to ASTM D412-64T at the same temperature employed to crosslink said admixture.